In the prior art, various approaches have been taken for the implementation of flush antenna structures. Various cavity enclosed antenna structures are extant in the prior art and any of these could be considered relevant to flush mounted air/spacecraft antennas, whether or not this prior art was developed for air/spacecraft employment.
Typical of the prior art cavity-type antennas are the devices shown in U.S. Pat. Nos. 3,836,976; 3,740,754 and 3,789,416. In U.S. Pat. No. 3,740,754, a turnstile antenna within a cup-like cavity is disclosed. The turnstile elements are bars or tubes self-supported from a central feed structure. It could be said that a radome might be affixed over the open cup and the device thereby converted to a flush mounted antenna by installing it in a corresponding opening in the skin of an air/space vehicle. The radiating elements of U.S. Pat. No. 3,740,754 would be unsuitable for the severe environment of air/space vehicle service, since in addition to air friction heating, shock and vibration are encountered. The discrete tubular or rod-like elements of that reference are likely to be unable to resist such shock and vibration and therefore its structure would be generally unsuitable for the application.
In U.S. Pat. No. 3,789,416, another turnstile antenna structure is shown mounted within a cup-like housing similar to the configuration of U.S. Pat. No. 3,740,754 in that its elements are mounted from a central feed. This device of U.S. Pat. No. 3,789,416 would be no more able to perform satisfactorily in the air/space vehicle application than would the apparatus of U.S. Pat. No. 3,740,754. Still further, both of these prior art devices would be relatively expensive to manufacture. Many metal forming steps and jig assembly appear necessary for either.
Concerning the so-called turnstile antenna configuration, it should be noted that this is a well-known concept in this art. It basically involves dipoles or colinear pluralities of dipoles in two orthogonal arrangements. Separate feeds permit separate excitation control and phasing for radiation pattern selection or polarization diversity or agility.
In U.S. Pat. No. 4,132,995, a cavity backed, slot-radiator antenna with an orthogonal, printed-circuit, feed strip is disclosed. While this disclosure shows the use of a printed circuit strip as a feed element, the actual radiator is a slot in a conductive sheet on the opposite side of the planar substrate sheet vis-a-vis the said feed element, facing into the cavity on one side and through the dielectric substrate and a radome sheet into the antenna aperture on the other side. No "turnstile" combination is provided by U.S. Pat. No. 4,132,995 and the radiation pattern is roughly a fixed cardiod.
Considering the use of the device of U.S. Pat. No. 4,132,995 as a flush antenna for very high speed, air/space, reentry type vehicles it becomes immediately apparent that the flat plane of the microstrip feed element would be separated from the high vehicle skin temperature induced by atmospheric reentry by only the relatively thin randome cover.
Other prior art is extant describing shaped, printed circuit dipoles and other printed radiators and the materials and processes for applying such printed circuit elements on a dielectric substrate, as for example by photolithography or selective etching, are now well understood by those of skill in this art.
Examples of microstrip (printed circuit) dipole and other radiators on dielectric substrates are disclosed in U.S. Pat. Nos. 4,012,741; 4,067,016; 4,072,951 and 4,155,089.